High voltage testing specifications call for the measurement of high voltage using either root mean square (RMS) voltage, which is equivalent to a direct current (DC) heating effect, or peak-scaled-to-RMS voltage (i.e., peak/(.sqroot.2)) for the measurement of high voltage. The two measurements are identical if the high voltage signal being measured is a perfect sine wave. In a practical high voltage system, however, the high voltage signal is not a perfect sine wave and the two measurements disagree with each other. Whether one of the two measurements is used over the other measurement depends on the tests being performed. The measurement of both of these parameters using purely analog processes introduces inaccuracy into the high voltage system.
In a voltage measurement system, the height of pulses in a signal is considered. Amplification is generally performed before the pulses are measured. For example, a band-limited amplifier is typically used to increase the magnitude of the pulses. Pulse height is then measured using an analog peak detection system. With reference to FIG. 1, an analog peak detector 2 uses a comparator 4 to charge a capacitor 6 through a diode 8. As long as the voltage on the capacitor 6 is less than the voltage of the pulse, the capacitor 6 is charged by the comparator 4. Once the voltage on the capacitor 6 exceeds that of the pulse, charging is stopped. The output voltage of the analog peak detector 2 at this point is equal to the highest voltage that has occurred on the input 3. When a data acquisition system is used to detect and measure individual pulses, the detector 2 must be reset by discharging the capacitor 6 after a pulse has been measured in order to be ready for the next pulse.
It is difficult to configure an analog peak detector of this type to be accurate. The analog peak detector relies on a non-linear feedback loop, which is dependent very heavily on the characteristics of the peak detection system. The delay around the peak detection system, particularly through the comparator 4 and an output buffer, causes the voltage of the output of the analog peak detector to lag fractionally with respect to the input, resulting in an overshoot on the output. The magnitude of this overshoot is typically non-linear with voltage, thereby limiting the accuracy of the system.
It is also necessary with an analog peak detector to compromise on the capacitor 6 used for peak detection. The voltage on the capacitor 6 tends to "droop" once the comparator 4 has stopped charging the capacitor because of leakage currents in the system. This introduces an uncertainty in the measurement, since the voltage decreases by some amount before the magnitude is measured. This effect can be limited by using a larger capacitance. A larger capacitance, however, requires more current for charging, resulting in a lower rate of change of voltage and limiting the maximum frequency that can be used for the amplifier. Thus, loop delays in the analog peak detection system are increased. A further complication is introduced when a reset is required on the analog peak detector. Charge injection from a reset switch can result in offsets on the output of the analog peak detector 2, which further limits the accuracy of the partial discharge measurement system.
Accordingly, a need exists for a voltage measurement system which allows for more accurate peak detection. In addition, a need exists for a voltage measurement system which permits concurrent measurement of peak voltage and RMS voltage. As stated previously, whether one of the two measurements is used over the other measurement depends on the tests being performed. The measurement of both of these parameters using purely analog processes introduces inaccuracy into the high voltage system. For example, to perform peak measurement, the peak capture circuit is designed to provide sufficient accuracy. To perform RMS measurement, on the other hand, design issues such as the long settling interval required for a converter to settle to a final value and a slow response to changes in the input are addressed. Use of an analog system for voltage feedback in a closed loop control system is therefore undesirable.